ESP8266: Connecting to MQTT broker

The objective of this post is to explain how to connect the ESP8266 to a MQTT broker, hosted on CloudMQTT.

Introduction

The objective of this post is to explain how to connect the ESP8266 to a MQTT broker. If you are not familiar with the protocol, you can read more about here.

Although this example should work fine with other brokers, we will assume that the broker will be hosted on CloudMQTT.

Since CloudMQTT has a free plan, we can just create an account and test it. Setting an account is really simple and it’s outside the scope of this post. You can check here how to do it and how to create a broker instance.

After completing the procedure, check the instance information page, which should be similar to the one shown in figure 1. The important credentials that we will be using on the ESP8266 code are the server, the user, the password and the port.

Figure 1 – CloudMQTT instance information.

In the ESP8266 side, we will be using an MQTT that supports the ESP8266, called PubSubClient. The library can be installed via Arduino IDE library manager. The coding shown here is based on the examples provided in the library, which I encourage you to try.

Regarding the hardware, the tests shown on this tutorial were performed using a ESP8266 NodeMCU board.

The code

First, we start by including the libraries needed for all the functionality. We need the ESP8266WiFi library, in order to be able to connect the ESP8266 to a WiFi network, and the PubSubClient library, which allows us to connect to a MQTT broker and publish/subscribe messages in topics.

#include <ESP8266WiFi.h>
#include <PubSubClient.h>

Then, we will declare some global variables for our connections. Naturally, we need the WiFi credentials, to connect to the WiFi network. You can check here a previous post explaining in detail how to connect to a WiFi network using the ESP8266.

We will also need the information and credentials of the MQTT server. As explained in the introduction, you will need to know the server address, the port, the username and the password. Use your values in the variables bellow.

After that, we will declare an object of class WiFiClient, which allows to establish a connection to a specific IP and port [1]. We will also declare an object of class PubSubClient, which receives as input of the constructor the previously defined WiFiClient.

The constructor of this class can receive other number of arguments, as can be seen in the API documentation.

WiFiClient espClient;
PubSubClient client(espClient);

Now, moving for the setup function, we open a serial connection, so we can output the result of our program. We also connect to the WiFi network.

Next, we need to specify the address and the port of the MQTT server. To do so, we call the setServer method on the PubSubClient object, passing as first argument the address and as second the port. These variables were defined before, in constant strings.

client.setServer(mqttServer, mqttPort);

Then, we use the setCallback method on the same object to specify a handling function that is executed when a MQTT message is received. We will analyse the code for this function latter.

client.setCallback(callback);

Now, we will connect to the MQTT server, still in the setup function. As we did in the connection to the WiFi network, we connect to the server in a loop until we get success.

So, we do a while loop based on the output of the connected method called on the PubSubClient, which will return true if the connection is established or false otherwise.

To do the actual connection, we call the connect method, which receives as input the unique identifier of our client, which we will call “ESP8266Client”, and the authentication username and password, which we defined early. This will return true on connection success and false otherwise

In case of failure, we can call the state method on the the PubSubClient object, which will return a code with information about why the connection failled [2]. Check here the possible returning values.

To finalize the setup function, we will publish a message on a topic. To do so, we call the publish method, which receives as input arguments the topic and the message. In this case, we will publish a “Hello from ESP8266” message on the “esp/test” topic.

client.publish("esp/test", "Hello from ESP8266");

Then, we will subscribe to that same topic, so we can receive messages from other publishers. To do so, we call the subscribe method, passing as input the name of the topic.

The callback function

After the initialization, we need to specify the callback function, which will handle the incoming messages for the topics subscribed.

The arguments of this function are the name of the topic, the payload (in bytes) and the length of the message. It should return void.

In this function, we will first print the topic name to the serial port, and then print each byte of the message received. Since we also have the length of the message as argument of the function, this can be easily done in a loop.

The main loop

In the main loop function, we will just call the loop method of the PubSubClient. This function should be called regularly to allow the client to process incoming messages and maintain its connection to the server [2].

Testing the code

First, make sure the MQTT server is running. Then, to test the code, just upload it and run it on your ESP8266. Open the Arduino IDE serial console, so the output gets printed.

Upon running, the ESP8266 will send the “Hello from ESP8266” message, which will not be printed on the serial. After that, the ESP8266 subscribes the same topic to which the hello message was sent.

If any other producer sends a message to the “esp/test” topic, then it will be printed in the serial console, as shown in figure 2.

Figure 2 – Messages sent to the “esp/test” topic.

For this tutorial, I used MQTTlens, a Google Chrome application, which connects to a MQTT broker and is able to subscribe and publish to MQTT topics [3]. This is a very useful application that I really recommend for this type of tests.

For the test, MQTTlens was subscribing the “esp/test” topic before connecting the ESP8266. As seen in figure 3, the “Hello from ESP8266” message was printed. After that, two hello messages were sent by MQTTlens, which can be seen in the same figure. The messages sent are the ones shown in figure 2, which were received by the ESP8266, as expected.

Note however that it is not clear if the library is able to verify the digital certificate of the broker. So, even if it works with SSL, it is kind of not that much useful if it is not able to perform that validation because you will not be able to confirm the identity of the server, which is a security flaw.

Hellow
i uploaded the full code upthere to my nodemcu, after sucsessfully aploading i can read on the serial monitor that my nodemcu is succesfully connected to the wifi and after that to the mqtt server. “connected” 🙂
after this there is no subscribed message received, my node is also not publishing messages to the server (i check on websocket there is no message received from client!!)
i tried sending message to the client from the web socket, but client (my nodemcu) is not receiving the message!!
i over checked if there is a difference in the spelling but nothing is missed!! so what can you say for me? hope you reply soon!!

At the time I haven’t tested more than one topic, but my guess is that you can call the subscribe method multiple times for different topics and then in the callback function use the topic argument to know from where the message was originated.

Hi antepher. my name is arafath baig, I am using esp8266 generic module in my project and i am using the example code of esp8266 mqtt and i am getting an error of mqtt not declared.
will you please help me. Give me your email id so that i can send you my code. Thank You.

I’ve not been using AT commands in a long time, so I’m not sure what you have available these days.

Nonetheless, it seems that you are trying to establish a raw TCP connection to the MQTT broker, which will not work.

MQTT is a protocol built on top of TCP/IP, so you would have to respect the protocol when sending the data.

If you want to send stuff to a MQTT topic using a TCP connection it pretty much means that you would have to implement MQTT on top of it yourself, which I really don’t recommend unless you have a lot of experience with the protocol and how networking works.

On top of that, since you are using AT commands, the task would be even harder.

My recommendation is either to try to find an AT command firmware that supports MQTT (not sure if it exists, I know that the UART OBLOQ,a device based on the ESP8266, has such firmware, so maybe you can try to use it) or program the ESP8266 using, for example, the Arduino core, and use a library that already implements the MQTT protocol.

I am working with MQTT for a while now. It is working very well but I have one small issue that I can not solve. Maybe you have an idea about the following issue.

It is an ESP8266-01 with a button and a LED. Pushing the button would change the state of the LED and send the info to the MQTT broker.
If the device, the ESP8266-01 lost it’s connection with the broker, it try’s to reconnect but if that fails it wait 5 seconds before retrying. If at that time I try to push the button, it will not work. All logic, but when the internet is down for a long time it is impossible to change the state of the LED.

I am using PubSubClient.h with the Arduino IDE.
Just wondering how you would deal with this but if this question is not allowed here, just ignore it please.

That’s a strange issue since for what I’ve understood from your system, sending the data to the broker is a side effect of clicking the button.

So, connecting the LED should be independent of if the Internet connection is up or not, right?

What I would do would be setting an interrupt to detect the click of the button.

Since interrupts need to run fast, I would simply set a flag to true and then in the main loop check the value of the flag.

If it was true, I would turn on the LED immediately and then check if I had internet connection + broker connection. If I had, I would then send the message to the broker.

That way, the LED should keep working independently of internet connection.

Now, one thing you need to figure out in your code is that if the Internet goes down and then goes up again, the ESPis automatically reconnecting. I don’t recall what was the behavior since I’ve not been working with the ESP8266 for a while, but I think that there was a function to set the ESP8266 to auto reconnect when the connection was lost.

On top of that, you need to check if, even if the internet reconnects, the connection to the MQTT broker persists.

Unfortunately I don’t know how the MQTT library behaves, if it can recover automatically from the internet going down or if it needs to be explicitly reconnected.

My recommendation is to run some isolated tests of that behavior and remove the led logic of the equation to figure that out.

For example, you can develop a simple program that sends a mqtt message every 30 seconds, turn off your router for a while and then reconnect it back and see if the messages start getting received again.

In terms of software development, it’s a good idea to test things separately before joining them in the final system.

That way, you can figure out more easily where the problem is during debugging.

Feel free to ask your questions 🙂

Unfortunately due to time constraints I cannot provide implementation code or look into extensive pieces of code, but I always try to give a conceptual help 🙂 Also, if I don’t know the answer, other readers of the blog may be able to help.

Hope this helps getting you in the right track and let us know if you were able to solve the issue 🙂

Hi Nuno Santos,
No worries mate. I am not searching for implementation code (although it may come in handy sometimes, to be honest🙂) or want you to solve this issue just like that. I was just wondering what side to look to or some pointers.

Maybe I have to look at the library or the way it tries to reconnect.
Thanks and I really appreciate your response.
Best regards,
Ray.

Hi,
I have not change anything in your code. I am able to send a publish message to the websocket of cloudmqtt, but I am not able to subscribe to the broker. I even tried to subscribe to broker via mosquitto from windows but still no success. I have no idea what might be causing this issue. Thanks in advance. 🙂

Sorry for the late reply. I successfully communicated via MQTT using SIM900. There I used the websocket of cloud MQTT to publish and subscribe. And see live updates.
This could’ve been some temporary issue, I will try it again and let you know if it worked or not.